1. Field of the Invention
This invention relates to new, nonionically hydrophilically modified, olefinically unsaturated polyurethanes and to a water-dispersible coating composition based on a mixture of synthetic resins having unsaturated groups which contains the nonionically hydrophilic polyurethanes as emulsifier.
2. Description of the Prior Art
Water-dispersible polyurethanes containing double bonds are known and described in EPA 0,098,752 and DE-OS 2,936,039. The hydrophilicity of these known systems is based on the presence of ionic centers, generally carboxylate or sulfonate groups, containing as counterions either alkali metal cations or ammonium ions which remain in the coatings ultimately obtained from the systems and seriously affect their water resistance. In addition, the amines often present can cause yellowing of the lacquer coatings while any auxiliary solvents present can have an environment-polluting effect during the drying process.
DE-OS 3,829,588 describes water-dispersible allyl urethanes which may also contain unsaturated dicarboxylic acid units. Dispersibility in water is achieved through the s incorporation of a special combination of polyethylene glycols. However, the relatively high content of polyethylene glycols leads to binders with unsatisfactory lacquer film properties.
Another method of obtaining water-dilutable products is to use external emulsifiers. Thus, according to U.S. Pat. No. 4,070,323 for example, acryloyl-functional polyurethanes are dispersed in water by means of anionic or cationic oil-in-water emulsifiers (for example sodium lauryl sulfate). These emulsifiers are not incorporated in the lacquer film during the radical crosslinking reaction. This considerably reduces the degree of water resistance of the lacquer films which could otherwise be achieved.
DE-OS 3,900,257 describes nonionically hydrophilic polyurethanes containing (meth)acryloyl groups and their use as reactive emulsifiers for nonwater-dispersible urethane (meth)acrylates. However, only a limited number of synthetic resins can be emulsified with these emulsifiers. For example, they are unable to emulsify styrene-free, unsaturated polyester resins.
Accordingly, an object of the present invention is to disperse radical-curable synthetic resins, more particularly unsaturated polyester resins, in water without incurring any of the disadvantages of the prior art.
This object may be achieved with the hydrophilic polyurethanes according to the invention which are described in more detail hereinafter and by their use in accordance with the invention which is also described in more detail hereinafter.